Fluorescence detection of single guest molecules in ultrasmall droplets of nonpolar solvent

We have investigated emissive behaviours of individual perylenebisimide derivatives, N,N'-dipropyl-1,6,7,12-tetrakis(4-tert-butylphenoxy)-3,4,9,10-perylenetetra-carboxydiimide (BP-PDI), in single ultrasmall droplets of n-octane at room temperature by using confocal and wide-field microscopic techniques. Single BP-PDIs in the small droplets show no distinguishable blinking in the time courses of fluorescence intensity. This is attributed to small probabilities of the formation of the long-lived ionized state leading to the off-state of the fluorescence. Temporal change in the degree of polarization of fluorescence and wide-field fluorescence images indicated short-time adsorption of the fluorescent molecules at the interfaces between n-octane and watery environments. Fluorescence correlation spectroscopy revealed that the adsorption/desorption processes took place at least in two different time scales, probably due to the difference in the adsorption geometry and/or in the interaction, such as van der Waals interaction and hydrogen bonding, between the dye and the interface.